The recovery of uranium from wet-process phosphoric acid has been the subject of prior study. Most marine phosphate rock contains from about 0.2 to 0.4 pounds of uranium per ton. Thus, the annual production of phosphate rock, on the order of about 40-50 millions tons yearly, represents several hundred thousand pounds of uranium.
In the wet-process for producing phosphoric acid, phosphate rock is treated with sulfuric acid in the presence of recycle phosphoric acid thereby precipitating calcium sulfate and releasing phosphoric acid. It is found that nearly all of the uranium contained in the phosphate rock is dissolved by sulfuric acid and remains in the filtrate of the crude phosphoric acid. It has been found that the filtrate of crude phosphoric acid contains from about 70% to about 90% of the uranium in the original phosphate rock.
To recover the minute uranium content of the crude wet-process phosphoric acid requires special processing techniques. These techniques are described in the Kirk-Othmer Encyclopedia of Chemical Technology, Second Edition, volume 21, pages 1 to 36, which are hereby incorporated by reference. Recovery of uranium from crude phosphoric acid by solvent extraction requires a solvent which is immiscible with water and which has a high affinity for uranium. These solvents extract uranium from crude phosphoric acid by the formation of a complex with the uranium. This uranium complex is soluble in excess solvent. In the usual case, the immiscible solvent which has a high affinity for uranium is dissolved in an organic carrier which is immiscible with water, such as a hydrocarbon, or substituted hydrocarbon, or an ether, alcohol or ketone. Kerosene, which is a mixture of hydrocarbons, is a preferred immiscible organic carrier. The uranium is transferred from the crude phosphoric acid to the immiscible organic solvent, and further refined and purified by techniques which are well known to the art.
The uranium-free phosphoric acid contains residual amounts of the immiscible organic carrier which must be removed. Most of the residual organic carrier is recovered by settling, that is, allowing the phosphoric acid to stand until the immiscible carrier forms a separate layer which is physically removed, e.g., by decanting. However, even after settling, the organic carrier level in acid after solvent extraction for uranium remains at about 500 to about 1000 ppm by weight. For many purposes it is essential to further reduce organic carrier level in the phosphoric acid in order, for example, not to damage rubber linings in subsequent processing equipment. Prior techniques to further reduce the amount of organic carrier in phosphoric acid after solvent extraction uranium recovery have involved settling the phosphoric acid over extremely long periods of time, aeration, and/or flotation techniques all of which are either excessively time consuming or otherwise economically unattractive.